686
FXUS63 KFGF 271947
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
247 PM CDT Thu Jun 27 2024

.KEY MESSAGES...

- There is a level 1 out of 5 risk for severe thunderstorms for
  all of eastern North Dakota tonight.

- There is a level 1 out of 5 risk for severe thunderstorms across
  the region Friday afternoon and evening.

- There is a low chance for severe thunderstorms again Monday
  across the region.

&&

.DISCUSSION...
Issued at 247 PM CDT Thu Jun 27 2024

...Synopsis...

Ridging is propagating eastward, with a mid/upper low
propagating eastward from the Pacific NW. Ensemble guidance in
good agreement depicting the upper level pattern to remain
progressive through the period. Ensemble members do show
variances with timing, track, strength of each passing mid level
trough and associated surface low/front evolution which leads
to variances in timing, location, and magnitude of any severe
impacts or heavy rain. Temperatures vary with passing shortwave
ridges/troughs with a tendency for below average temperatures at
time (notably this weekend behind the trough on Friday). Each
passing trough carries at least some potential for severe
thunderstorms, with details tied to the mentioned uncertainties.

...Severe threat overnight...

Mid-level warm air advection currently propagating into the
region leading to scattered showers. Instability is very weak,
so not much for thunder at this point, although do anticipate at
least isolated thunder at times. This activity will continue to
move northeast into the evening hours.

The more widespread coverage overnight will be associated with
stronger 850mb WAA associated with a stronger low level jet.
With this stronger forcing, SPC indicates the marginal risk for
severe severe storms (at least for eastern North Dakota),
although instability quickly wanes after sunset - and forcing
arrives after sunset. Given lack of instability most convection
will be weaker, although strength of forcing and strong deep
layer shear will lead to a stronger storm or two. Primary
threats remain hail, and wind where momentum transfer of heavy
rain/elevated downdrafts can mix down higher gusts in the higher
low level shear environment.

Will also need to monitor for stronger wake low winds as the
convection tonight propagates eastward - the latest runs of the
HRRR indicate this potential. At this point predictability is
very low - especially considering the weaker nature of the
anticipated convection.

...Severe threat Friday afternoon/evening...

Deformation zone is favored by latest trends to shift north through
Friday morning towards the US/Canada border, lingering into the
afternoon. A mid level dry slot is shown to allow for clearing
across much of the region. Latest guidance in good agreement
indicating the main axis of instability will be east of the CWA
by mid-afternoon. However, there is a signal for MLCAPE up to
1000 J/KG along and just ahead of an inverted trough which
connects back toward the mid-upper low which will still be
approaching from the west. This will tend to be in a post
frontal shear environment (backing low levels) but deep layer
shear remains high (top heavy shear profiles). Due to the
strong effective shear we could see low topped supercell
development. If clearing and enough instability builds along and
ahead of the inverted trough, low level vorticity along with
stronger low level lapse rates/CAPE and slower propagating
frontal boundary could lead to a conditional tornado threat.
Given lower freezing levels, hail up to 1.5 or so inches also
would be a threat.

...Severe threat Monday....

Another mid/upper wave follows a very similar track and evolution
as the system Friday, with a period of widespread showers or
embedded thunderstorms. There is a signal for a deeper warm sector
building into our region within most clusters, and higher probs for
SB CAPE to exceed 1000 J/KG during the peak heating/diurnally
favored period of the afternoon/evening. CSU GEFS based machine
learning (15-30% severe probs) are supported by 100% chance for
greater than 55 NBM CWASP and 50-60% probability for NBM CWASP
values exceeding 65. That aligns with a threat for isolated to
scattered severe convection. This is still farther out, so
details/impacts are still inherently uncertain.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1237 PM CDT Thu Jun 27 2024

Area of MVFR cigs across central North Dakota will continue to
slowly approach the region, and have attempted to time these
into each site. For the most part, anticipate these lower cigs
late afternoon or evening. Scattered showers (with a very low
chance for embedded thunder) anticipated along with the lower
cigs, although not sure of coverage so kept VCSH. More
widespread showers/thunder arrive later this evening and
overnight.

&&

.FGF WATCHES/WARNINGS/ADVISORIES...
ND...None.
MN...None.

&&

$$

DISCUSSION...TG/DJR
AVIATION...TG